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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="move.composition_inheritance"></a><a class="link" href="composition_inheritance.html" title="Composition or inheritance">Composition or inheritance</a>
</h2></div></div></div>
<p>
      For classes made up of other classes (via either composition or inheritance),
      the move constructor and move assignment can be easily coded using the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span></code>
      function:
    </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Base</span>
<span class="special">{</span>
   <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span>

   <span class="keyword">public</span><span class="special">:</span>
   <span class="identifier">Base</span><span class="special">(){}</span>

   <span class="identifier">Base</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Base</span> <span class="special">&amp;/*</span><span class="identifier">x</span><span class="special">*/)</span> <span class="special">{/**/}</span>            <span class="comment">// Copy ctor</span>

   <span class="identifier">Base</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span> <span class="comment">/*x*/</span><span class="special">)</span> <span class="special">{/**/}</span>     <span class="comment">// Move ctor</span>

   <span class="identifier">Base</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span> <span class="comment">/*x*/</span><span class="special">)</span>
   <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;}</span>                      <span class="comment">// Move assign</span>

   <span class="identifier">Base</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">Base</span><span class="special">)</span> <span class="comment">/*x*/</span><span class="special">)</span>
   <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;}</span>                      <span class="comment">// Copy assign</span>

   <span class="keyword">virtual</span> <span class="identifier">Base</span> <span class="special">*</span><span class="identifier">clone</span><span class="special">()</span> <span class="keyword">const</span>
   <span class="special">{</span>  <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">Base</span><span class="special">(*</span><span class="keyword">this</span><span class="special">);</span>  <span class="special">}</span>

   <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Base</span><span class="special">(){}</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">Member</span>
<span class="special">{</span>
   <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Member</span><span class="special">)</span>

   <span class="keyword">public</span><span class="special">:</span>
   <span class="identifier">Member</span><span class="special">(){}</span>

   <span class="comment">// Compiler-generated copy constructor...</span>

   <span class="identifier">Member</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Member</span><span class="special">))</span>  <span class="special">{/**/}</span>      <span class="comment">// Move ctor</span>

   <span class="identifier">Member</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Member</span><span class="special">))</span>   <span class="comment">// Move assign</span>
   <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>  <span class="special">}</span>

   <span class="identifier">Member</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">Member</span><span class="special">))</span>   <span class="comment">// Copy assign</span>
   <span class="special">{/**/</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>  <span class="special">}</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">Derived</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Base</span>
<span class="special">{</span>
   <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span>
   <span class="identifier">Member</span> <span class="identifier">mem_</span><span class="special">;</span>

   <span class="keyword">public</span><span class="special">:</span>
   <span class="identifier">Derived</span><span class="special">(){}</span>

   <span class="comment">// Compiler-generated copy constructor...</span>

   <span class="identifier">Derived</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span>             <span class="comment">// Move ctor</span>
      <span class="special">:</span> <span class="identifier">Base</span><span class="special">(</span><span class="identifier">BOOST_MOVE_BASE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">x</span><span class="special">)),</span>
        <span class="identifier">mem_</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">mem_</span><span class="special">))</span> <span class="special">{</span> <span class="special">}</span>

   <span class="identifier">Derived</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span>  <span class="comment">// Move assign</span>
   <span class="special">{</span>
      <span class="identifier">Base</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_MOVE_BASE</span><span class="special">(</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">x</span><span class="special">));</span>
      <span class="identifier">mem_</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">mem_</span><span class="special">);</span>
      <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
   <span class="special">}</span>

   <span class="identifier">Derived</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span>  <span class="comment">// Copy assign</span>
   <span class="special">{</span>
      <span class="identifier">Base</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">x</span><span class="special">);</span>
      <span class="identifier">mem_</span>  <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">mem_</span><span class="special">;</span>
      <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
   <span class="special">}</span>
   <span class="comment">// ...</span>
<span class="special">};</span>
</pre>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
        Due to limitations in the emulation code, a cast to <code class="computeroutput"><span class="identifier">Base</span>
        <span class="special">&amp;</span></code> is needed before moving the
        base part in the move constructor and call Base's move constructor instead
        of the copy constructor.
      </p></td></tr>
</table></div>
<p>
      Each subobject will now be treated individually, calling move to bind to the
      subobject's move constructors and move assignment operators. <code class="computeroutput"><span class="identifier">Member</span></code> has move operations coded (just like
      our earlier <code class="computeroutput"><span class="identifier">clone_ptr</span></code> example)
      which will completely avoid the tremendously more expensive copy operations:
    </p>
<pre class="programlisting"><span class="identifier">Derived</span> <span class="identifier">d</span><span class="special">;</span>
<span class="identifier">Derived</span> <span class="identifier">d2</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">d</span><span class="special">));</span>
<span class="identifier">d2</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">d</span><span class="special">);</span>
</pre>
<p>
      Note above that the argument x is treated as a lvalue reference. That's why
      it is necessary to say <code class="computeroutput"><span class="identifier">move</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>
      instead of just x when passing down to the base class. This is a key safety
      feature of move semantics designed to prevent accidently moving twice from
      some named variable. All moves from lvalues occur explicitly.
    </p>
</div>
<div class="copyright-footer">Copyright © 2008-2024 Ion Gaztanaga<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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